Successful exploit of the vulnerability would have left PCs wide open to attack

Microsoft on Tuesday warned customers that its malware detection engine, used in a wide range of its products including Security Essentials and Windows Defender, could be disabled if an attacker sent a malformed file as an email attachment.

Along with the security alert, Microsoft issued an update to patch the vulnerability.

A successful attack would leave a Microsoft-guarded PC wide open to subsequent exploits, the company warned.

"An attacker who successfully exploited this vulnerability could prevent the Microsoft Malware Protection Engine from monitoring affected systems until the specially crafted file is manually removed and the service is restarted," Microsoft said in an advisory yesterday.

The latter is a PC-cleaning tool updated monthly that seeks out and destroys selected malware. Windows Defender is the security software packaged with Windows 8 and Windows 8.1, while the free Security Essentials has been a mainstay for many consumers and even small businesses running Windows 7.

Microsoft said it had no received no reports that the vulnerability had been exploited in the wild. But the bug was serious, marked "important" -- Microsoft's second-highest threat label.

It wasn't difficult to see why: If the anti-malware software was set to automatically scan every incoming file, as most are, an attacker could cripple defenses simply by attaching a specially-crafted file to an email message. Most customers, especially consumers, would have no idea that their PCs were now unguarded.

The update Microsoft released at the same time it published the advisory will be automatically delivered to the malware detection engine in the next 48 hours, Microsoft said.

A Google security engineer, Tavis Ormandy, reported the vulnerability to Microsoft. According to Ormandy, he found the flaw in the JavaScript interpreter used by Microsoft's anti-malware engine.